Serum or plasma concentration of hepatitis B surface antigen (HBsAg) by immunoassay

Why This Test Is Done

Your healthcare provider may order this test if you have symptoms of hepatitis B, such as fatigue, nausea, abdominal pain, or jaundice (yellowing of the skin and eyes). It may also be done as part of a routine check if you’re at higher risk, like having multiple sexual partners, receiving blood transfusions, or having a job that involves frequent exposure to blood.

What to Expect Before the Test

No special preparation is usually needed. You may be asked to fast before the test, but this depends on the lab’s instructions. Bring any recent test results or medical information that might be relevant to your provider.

What the Test Results Mean

A positive result means the virus is actively replicating in your body, and you may have hepatitis B. A negative result suggests no current infection, but you could still be at risk if exposed recently. Follow-up testing may be needed to monitor your health.

Possible Side Effects

The most common side effect is a mild bruise or soreness at the needle site. Rarely, there may be a reaction to the blood draw, such as swelling or redness.